"It is neither cold nor hot in fall."
My instinct was to leave out the 'ye3' and make the sentence parallel the 'you4...you4' structure. '...bu neng bu re' Is that valid?
Actually, the pinyin is "bu leng bu re". You are expecting "不冷不熱". Even though your proposed translation is literally "not cold not hot", this instead transforms the given sentence to "It (The temperature) is alright (in the middle)". The more casual translation would then be "It is nice!" if you do not include 也 in the translation.
For the Duolingo translation, it is alright as it is.
I think that is acceptable (3 semesters of university classes) but would like to hear from a native speaker what is correct and/or natural.
Just to avoid confusion, 又不冷又不熱 is not correct.
不冷不熱 or 不熱不冷 are both good phrases (personally I think the latter is a little more natural); However it is more suitable to be followed by another clause, or it would feel like an unfinished sentence. e.g. 秋天不熱不冷，感覺很舒服。